Various investigations have hitherto been conducted for the purpose of accelerating curing of an epoxy resin. Examples of such a technique include a technique described in Patent Document 1. According to the same document, it is described that by using a phosphonium-based compound, it is possible to accelerate curing of an epoxy resin, the fluidity during molding is excellent, high curing strength is exhibited, and curing can be performed even for a short curing time.